It has been common for air filters for automotive vehicles to be made of paper which is pleated to provide a large surface area for trapping dust particles etc., which are carried in the air passing to the internal combustion engine. Recently filters of plastics foam have become available, consisting of a single sheet of plastics foam impregnated with a substance (e.g. a plasticiser, or an engine oil) which serves to hold the particles which have become trapped by the filter, but these are of limited effectiness.